Anyway, as with all electronic kit, it could be just plain faulty, or alternatively a loose connection.
A solid reliable earth is as good a place to start as anywhere, followed by the supply voltage.
If memory serves, the Mcar interfaces with the Comand's fibre optics. This is an area I know little about. Speak to Comand Onlinehttps://www.commandonline.co.uk/ who are excellent and well regarded on this forum.